Vehicles have been developed to perform an idle-stop when idle-stop conditions are met and automatically restart the engine when restart conditions are met. Such idle-stop systems enable fuel savings, reduction in exhaust emissions, reduction in noise, and the like. Engines may be restarted from the idle-stop condition in response to a vehicle launch request from the operator, or restarted automatically, without receiving an operator input, for example, in response to engine operating parameters falling outside a desired operating range.
One example approach to shutting down the engine during an idle-stop condition includes shutting off a fuel supply to the cylinders, for example, by performing a deceleration fuel shut-off (DFSO). However, the inventors have recognized several potential issues with such an approach. As one example, following the fuel supply shut-off, the engine may not stop moving immediately and the pistons may continue to pump air through the engine as the engine coasts down. The air pumped through the engine exhaust system may saturate an exhaust catalyst with oxygen and/or reduce the catalyst temperature below its operative range. As such, either situation may necessitate extra fuel to recondition the catalyst during a subsequent restart. The extra fuel constitutes an increase in fuel consumption.
Another example approach for rapidly shutting down the engine during an idle-stop condition is illustrated by Kerns et al. in U.S. Pat. No. 7,159,561. Herein, a shutdown sequence is performed in two phases with a first phase where spark is retarded late into the power stroke, and a second phase where the spark is advanced into the compression stroke. The delayed spark in the first phase allows the engine to be slowed down while the combustion gases are delivered to the catalyst. The advanced spark in the second phase allows the engine to be rapidly stopped by causing a combustion event in the compression stroke. While the approach may address catalyst oxygen saturation and catalyst temperature issues, by putting spark in control, potential misfires or unintended combustions may arise due to ignition system errors and/or degradation. Such results may reduce the chances of a timely engine shutdown.
Further, if a driver has a change of mind while the engine is being shutdown (e.g., still spinning down) and wishes to immediately restart the engine, a desirable fast restart may not be possible. Specifically, the driver may have to wait for the engine to stop rotating completely before the engine starter can be re-engaged. As such, this may substantially increase the restart time and degrade the quality of the restart operation.
Thus, in one example, some of the above issues may be addressed by a method comprising, during an automatic engine idle-stop, turning off spark, and operating a first cylinder with a rich ratio of air to injected fuel richer than a rich flammability limit, and operating a second cylinder with a lean ratio of air to injected fuel leaner than a lean flammability limit. The method may further comprise mixing un-combusted exhaust from the first and second cylinders with exhaust, the exhaust mixture being substantially stoichiometric.
For example, engine shutdown may be expedited by engaging a starter without applying starter current and fueling the engine without spark during an idle-stop operation. By engaging the starter without applying the starter current, engine reversals during engine spin-down may be reduced. In one example embodiment, the engine shutdown may be performed when MAP is at or near atmospheric pressure (or barometric pressure BP). Alternatively, MAP may be adjusted towards BP, for example, using a throttle. As such, a substantially faster engine spin-down may be achieved by initiating the engine shutdown at BP, than at sub-atmospheric pressures.
By fueling the engine without spark during the shutdown, the combustion may occur at the catalyst and not in the cylinder or port. Consequently, an exothermic reaction may be generated at the catalyst to increase catalyst temperature just before engine stop, thereby prolonging the duration that the catalyst remains above the light-off temperature during engine-off. By fueling without spark, unintended combustion in the cylinder may be reduced. Combustion due to accidental sparks may be further reduced by fueling the engine outside the flammability limits of spark. For example, based on engine operating conditions, such as engine speed, engine temperature, and cylinder position, some cylinders may be fueled rich and over the flammability limit while some cylinders may be fueled lean and under the flammability limit. The amount of fuel injected into each cylinder (that is, a degree of leanness or richness) may be adjusted such that the air-fuel ratio of the final exhaust mixture directed through the catalyst is stoichiometric. Then, during a subsequent engine restart, the engine may be controlled based on the catalyst temperature and/or exhaust air-fuel ratio.
In this way, engine shutdown may be expedited, thereby reducing the amount of oxygen pumped through the catalyst. By reducing the amount of air pumped through the catalyst, the amount of fuel needed to condition the catalyst during the subsequent engine restart may be decreased, thereby improving fuel economy. By temporarily raising the catalyst temperature at the time of shutdown, the duration during engine-off that the catalyst remains above the light-off temperature may be extended. Further, by combusting at the catalyst, oxygen saturation of the catalyst may be reduced. In this way, the incidence of engine restart responsive to low catalyst temperatures may be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.